RFR 8051398: jvmti tests fieldacc002, fieldmod002 fail in nightly with errors: (watch#0) wrong location

[Coleen Phillimore]

Summary: Didn't handle NULL bcp for native methods

bcp is set to NULL in the interpreter frame for native methods.  x86 
generate_native_entry() contains a call_VM that sets the bcp address to 
the beginning of code, but sparc doesn't.  I don't think ppc does 
either.   The code I changed, doesn't handle a null bcp which 
post_field_access and post_field_modification tests call from JNI for a 
native method.

open webrev at http://cr.openjdk.java.net/~coleenp/8051398/
bug link https://bugs.openjdk.java.net/browse/JDK-8051398

Tested with jck vm/jvmti, jtreg, and NSK internal tests.  No test added 
because there's a test that already tests this.

Thanks,
Coleen